1. Field of the Invention
The invention is directed to a system for automatically switching lighting devices in vehicles.
2. Description of the Prior Art
One automatic light switching system, known from German Patent Disclosure DE 195 23 262 A1, has a sensor device, by which the light intensity in the surroundings of the vehicle is detected. The sensor device is connected to an evaluation device, by which the signals of the sensor device for the light intensity are compared with at least one threshold value and from that it is ascertained whether a change in the switching state of the lighting devices is necessary; that is, whether the lighting devices have to be switched on or can be switched off. In this known system, no distinction is made between light intensity originating from natural sunlight and light intensity originating from artificial light sources. For this reason, it can happen in the known system that the lighting devices of the vehicle are switched off or fail to be switched on when the vehicle is moving in a region of strong light intensity that originates in artificial light sources, as for instance in a tunnel or underground garage. In such areas, the lighting devices of the vehicle should, however, be switched on or remain switched on, which is not assured with the known-system.
The system of the invention has the advantage over the prior art that a distinction is made between the light intensity of a light component originating in natural sunlight and the light intensity of a light component originating in artificial light sources. This exploits the fact that artificial light sources are typically operated at alternating voltage, and thus as a function of the frequency of the alternating voltage, there are periodic changes in the intensity of the light emitted by the light sources, which reliably distinguish this light component from a light component originating in natural sunlight.